This invention relates to air conditioning systems which include both a hot deck for supplying heated air and a cold deck for supplying cooled air to a plurality of zones and, more particularly, to minimizing the heating and cooling costs in such a system.
A typical commercial building consists of a plurality of offices or rooms which, in the air conditioning industry, are called zones. Commercial buildings usually have two types of zones, exterior zones which are those located around the perimeter of the building and are affected by solar radiation, outdoor temperature and wind, and interior zones which, because they have no walls or windows exposed to the outdoors, are not affected by solar radiation, outdoor temperatures and wind.
Because the exterior zones are affected by solar radiation, wind and outdoor temperature conditions, the heating and cooling of such exterior zones are affected by seasonal changes. Specifically, during the winter months the exterior zones are heated and during the summer months the exterior zones are cooled. Interior zones are not affected by seasonal changes and are usually cooling loads during both winter and summer. The air conditioning system in such a building must, therefore, be capable of supplying both cooling air and heating air.
One common system for the supply of air conditioned air to such buildings is the double duct system having a hot deck for supplying heated air and a cold deck for supplying cooled air to the zones. Such air conditioning systems are controlled in a variety of ways. For example, both the hot deck and the cold deck may be connected to each zone and the air from each deck may be mixed in a ratio to maintain the temperature of that zone at the desired setting. The temperature of the air issuing from the cold deck is controlled at a point to satisfy the zone needing the most cooling, and the air from the cold deck is then mixed with the air from the hot deck in ratios to satisfy the cooling needs of the other cooling zones. Likewise, the temperature of the air issuing from the hot deck is controlled at a point to satisfy the zone needing the most heat and the air issuing from the hot deck is mixed with the air issuing from the cold deck in ratios to satisfy the other heating zones. Energy is conserved in this manner because the temperature of the hot deck is not hotter than is sufficient to meet the needs of the zone requiring the most heating and the temperature of the cold deck is not colder than is sufficient to meet the needs of the zone requiring the most cooling.
Both the hot deck and the cold deck are supplied with air from a supply duct which derives its air from, typically, a return air duct which returns air from the zones of the building to the air conditioning system and an outdoor duct which draws fresh air from the outside into the building. The return air duct and the outdoor air duct each have respective dampers for controlling the mixture of outdoor and return air supplied to the hot and cold decks. The supply duct may also have a cooling coil therein for cooling and dehumidifying the mixture of return air and outdoor air. To the extent possible, the outdoor and return air dampers and the cooling coil in the supply duct are controlled such that a minimum amount of cooling is done in the cold deck to satisfy the zone having the greatest cooling load. Thus, the mixed air is treated as a free source of cooling and ideally the temperature of this mixed air is controlled at a point which will require no further cooling in the cold deck. The fallacy of this approach is that heating can cost much more than cooling and the use of free cooling, therefore, can be very expensive in terms of heating costs. The present invention discards the free cooling approach and instead controls the temperature of the mixed air at a point which will minimize the total heating and cooling costs of the air conditioning system.